package LC;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * https://leetcode.com/problems/4sum/description/
 * Given an array S of n integers,
 * are there elements a, b, c, and d in S such that a + b + c + d = target?
 * Find all unique quadruplets in the array which gives the sum of target.
 * Note: The solution set must not contain duplicate quadruplets.
 * For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.
 * A solution set is:
 * [
 * [-1,  0, 0, 1],
 * [-2, -1, 1, 2],
 * [-2,  0, 0, 2]
 * ]
 */
public class LC_018_4Sum_Loop_TwoPointers {
    public static void main(String[] args) {
        int[] nums = {1, 0, -1, 0, -2, 2};
        int target = 0;
        List<List<Integer>> lists = fourSum(nums, target);
        System.out.println(lists.toString());
    }

    private static List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (nums == null || nums.length < 4) return result;
        Arrays.sort(nums);
        for (int i = 0; i < nums.length - 3; i++) {
            if (i != 0 && nums[i] == nums[i - 1]) continue;
            for (int j = i + 1; j < nums.length - 2; j++) {
                if (j != i + 1 && nums[j] == nums[j - 1]) continue;
                int k = j + 1;
                int l = nums.length - 1;
                while (k < l) {
                    int sum = nums[i] + nums[j] + nums[k] + nums[l];
                    if (sum < target) {
                        k++;
                    } else if (sum > target) {
                        l--;
                    } else {
                        result.add(Arrays.asList(nums[i], nums[j], nums[k], nums[l]));
                        k++;
                        l--;
                        while (k < l && nums[l] == nums[l + 1]) l--;
                        while (k < l && nums[k] == nums[k - 1]) k++;
                    }
                }
            }
        }
        return result;
    }
}
